This type of image forming apparatus uniformly charges an image carrier surface, scans the image carrier surface with light beam to form an electrostatic latent image on the image carrier surface. Then, the image forming apparatus develops the electrostatic latent image on the image carrier surface to form a toner image on the image carrier surface, transfers the toner image from the image carrier to a recording paper sheet. Next, the image forming apparatus heats and applies pressure on the recording paper sheet to fix the toner image on the recording paper sheet.
The toner image is transferred from the image carrier to the recording paper sheet by sandwiching the recording paper sheet at a nip region between the image carrier and a transfer roller. Registration rollers are provided at an upstream side with respect to this nip region in a conveyance direction of the recording paper sheet. The registration rollers once stop the recording paper sheet to adjust a starting time point of conveying the recording paper sheet from the registration rollers so as to match the starting time point with a transfer timing of the toner image at the nip region.
Here, if the nip region, which is disposed between the image carrier and the transfer roller, if the recording paper sheet is displaced in a direction perpendicular to a paper sheet conveyance direction (a main-scanning direction), a printing position of the toner image on the recording paper sheet is displaced in the main-scanning direction. In view of this, a paper sheet position sensor is provided at an upstream side with respect to the registration rollers in the conveyance direction of the recording paper sheet. The paper sheet position sensor detects the conveyance position of the recording paper sheet in the main-scanning direction. Based on the conveyance position of the recording paper sheet, which is detected by the paper sheet position sensor, an image formation position of the toner image on the image carrier is corrected in the main-scanning direction to prevent the printing position of the toner image on the recording paper sheet from being displaced.
For example, Patent Literature 1 adjusts an image formation position of an image (a toner image) on an image carrier based on a preset main-scanning start adjustment position and a main-scanning start adjustment position detected by a side registration sensor (a paper sheet position sensor).
In Patent Literature 2, in the case where at least one of an amount of lateral displacement (a displacement amount in a main-scanning direction) and an amount of skew of a recording paper sheet, which are detected by a paper sheet position sensor (CIS), is larger than a first threshold (5 mm), printing of the recording paper sheet is stopped. Alternatively, in the case where at least one of the amount of lateral displacement and the amount of skew of the recording paper sheet is equal to or less than the first threshold and is larger than a second threshold (2 mm), the printing of the recording paper sheet is continued.
Further, Patent Literature 3 uses a displacement amount of the previous recording paper sheet detected by a paper sheet position sensor (CIS) to correct an image formation position on an image carrier. This prevents a displacement of an image on a recording paper sheet. Additionally, this technique calculates an average value of the displacement amounts of previous respective recording paper sheets and uses this average value to correct the image formation position on the image carrier. This prevents the displacement of the image on the recording paper sheet.
Patent Literature 4 switches a linear mode and a high speed correction mode. The linear mode corrects an image formation position on an image carrier using a displacement amount of the recording paper sheet of this time, which is detected by a paper sheet position sensor (CIS). The high speed correction mode corrects the image formation position on the image carrier using the displacement amount of the previous recording paper sheet detected by the CIS.
Thus, the image formation position on the image carrier is corrected in the main-scanning direction to prevent the displacement of the printing position of the image on the recording paper sheet or a plurality of correction methods are used according to the need.